The objective of the proposed work is to investigate the relationship between cell proliferation and neoplastic competence of perinatal lung tissue. Physiologic peaks of Type 2 alveolar cell proliferation will be determined using a combination of morphometric and autoradiographic techniques. Exposure of the Type 2 target cell to carcinogen will then be made during periods of high and low Type 2 cell proliferation and the number of lung tumors obtained during the different periods compared.